Pre amp hum in Ampeg GU12R

[jvm]

I have a '68 Ampeg GU12R with a loud hum at both cathode and plate of first gain stage. I can pull V1 and stop the hum. I have replaced the cathode bypass cap and tube with no change. Any help at all appreciated.

[Enzo]

I just bet...

I had one of these hummers once, and right behind the power switch area is a terminal strip, yes? Seems to me there was a wire from ground on the main cicuit board over to this strip or perhaps to the filter cap. Yeah, maybe that was it, the wire ran to the filter cap and then another wire from the filter cap common to the chassis ground. SO the whole preamp ground wound up sharing the ground return for the filter cap. This causes hum. By moving the wire to the chassis ground on the terminal strip (if i recall right) we no longer had the filter cap return sharing a ground. This simple moving of a four inch hunk of wire cured it, and apparently the amp had hummed like that its entire life.

See if that helps any, sorry I can't paint a clearer picture.

[jvm]

Thanks for the suggestion, Enzo. I moved the wire as you suggested and it did lower the hum somewhat. Any ideas how I might improve the grounding scheme on this amp. The amp sounds great, just hums loudly. I have replaced all filter caps, checked/cleaned all ground points including at input jacks. I have removed all tubes and still have the hum at cathode and plate of V1.

[Enzo]

If I had one in front of me, I could tell you just what I moved to where. If I recall there was a terminal strip and maybe an empty terminal, and I may have moved the ground connjection to the open spot. But to go from annoying hum to nice and quiet, all I had to do was move the ground for the main filter off the return wire from the preamp.

Memory is dim, but seems to me the stock amp had a ground wire from circuit board to the main filter cap ground then to chassis. And the cure was to change that to the board ground wire going to chassis separately from the cap ground wire. The changes I had to make would not alter the schematic any, just the wiring diagram.

[jvm]

I finally dug this Ampeg out of the "project" pile. I moved the standby switch ground from the board to a chassis lug. No more hum! Thanks for your help, Enzo!